This invention relates to a method for the preparation of thermally stable, polycrystalline silicon carbide ceramic fibers. The method described herein comprises converting ceramic fibers to the polycrystalline silicon carbide fibers by heating the fibers in an environment comprising a volatile sintering aid. The resultant fibers exhibit excellent mechanical strength at high temperatures and, thus, are useful, for example, as reinforcements for plastic, ceramic or metal matrices or for the formation of products such as high temperature insulation, belting, gaskets and curtains.
Several methods for the production of silicon carbide fibers have been developed. For instance, it is known that organosilicon polymers may be spun into fibers, infusibilized (cured) to prevent melting and ceramified at elevated temperatures. Such a process is used commercially to produce NICALON.TM. SiCO fibers and TYRANNO.TM. SiCOTi fibers from polycarbosilane and titanium-modified polycarbosilane, respectively. This method, however, does not produce high density polycrystalline fibers and it has been shown to introduce substantial amounts of oxygen into the fiber. When such fibers are heated to temperatures above 1300.degree. C., the oxygen volatilizes as CO and/or SiO causing weight loss, porosity and decreased tensile strength.
Methods for the preparation of thermally stable, polycrystalline silicon carbide fibers are also known in the art. For instance, U.S. Pat. No. 5,071,600 describes a process for forming such fibers in which polycarbosilane resin is spun into a green fiber, treated with boron, infusibilized and pyrolyzed at a temperature in excess of 1600.degree. C. under an inert atmosphere. It can be seen that the process of this reference is clearly different than that of the present invention in that the boron is incorporated at an early stage of the fiber formation process (e.g., before or during the cure) whereas the present invention describes incorporation at elevated temperatures during ceramification.
Smoak in U.S. Pat. No. 4,237,085 teaches the preparation of silicon carbide bodies by a process which comprises molding SiC powder and elemental carbon into the desired shape and heating the molded body in an atmosphere containing boron. The method and products of the reference, however, are clearly different than those taught herein.
The present inventors have now unexpectedly discovered that thermally stable, polycrystalline SiC fibers can be formed by the process of the present invention.